I found a response back in 2017 that seems to address this: topic14262.html"Topic <name> is referenced in the DITA map as "resource-only". Any cross reference to it will result in a broken link"
In particular, this statement:
My situation matches almost exactly: I have an xref with an href pointing to a dita topic. But my dita topic is not read only. The <name> topic appears in the PDF output.issued by Oxygen when you have an xref with an href pointing to "operator_conrefs.dita" or to a subelement inside it. As the "operator_conrefs.dita" is read-only and does not appear for example in the PDF output, all links to it would be broken
I compiled my PDF and tested out the links. The originating topic in question had several links flagged with this message. The weird thing is, one link is indeed disabled and does not work, while the other one works fine!